THURSDAY, Feb. 8, 2018 (HealthDay News) -- A powerful multiple sclerosis drug presents women with a tough dilemma if they would like to have children, a pair of new studies suggests.

Those who take Tysabri (natalizumab) to manage their MS are more likely to suffer a relapse during pregnancy if they stop taking the drug before they conceive, the first study found.

But if a woman remains on Tysabri while trying to get pregnant, her unborn child could face serious health risks, the second study showed.

Fetal exposure to Tysabri up to 12 weeks of gestation was associated with an increased risk of miscarriage, the Italian researchers discovered. Babies exposed to the drug in the womb also tended to have shorter length and lower weight at birth.

The researchers said their findings provide a path forward for couples who'd like to start a family.

"Our findings suggest that if women who take natalizumab for MS want to become pregnant, it may be best to continue treatment up until a pregnancy test is positive and then at that point discontinue use," lead researcher Dr. Emilio Portaccio said in a statement.

"While there is still a risk of increased disease activity, this course of action may lower that risk," said Portaccio, a neurologist with the Don Carlo Gnocchi Foundation in Florence.

Other MS experts are less certain, arguing that these studies simply provide more data for couples to consider when planning a family.

"I don't think decisions should be made based on any one study," said Kathy Costello, associate vice president of health care access for the National MS Society.

Multiple sclerosis is an autoimmune disease in which the body mistakenly attacks nerve cells. Tysabri helps control the disease by blocking immune cells from leaving the bloodstream and traveling into the central nervous system, where they can attack nerve cells and disrupt communication between brain and body, Costello said.

It's a drug typically prescribed for MS patients who haven't responded to other treatments or can't tolerate them, the researchers said.

During pregnancy, women usually see an easing of their MS symptoms, possibly because the immune system naturally backs off to protect the health of the fetus, Costello explained. However, risk of relapse returns following delivery and can be even sharper, particularly if the patient has stopped taking Tysabri.

On the other hand, regulatory agencies have recommended that women discontinue Tysabri for at least three months before conception, given studies that have linked the drug to increased risk of miscarriage, the authors added. The drug has also been linked to an increased risk for a rare, and sometimes fatal, brain infection.

To test the safety of Tysabri during pregnancy for both mother and child, Portaccio and his colleagues tracked 92 pregnancies in 83 women taking the drug. There ended up being 74 live births among the pregnancies.

The women were divided into two groups, depending on when they took their last dose of Tysabri.

Women who stopped taking Tysabri before their last menstrual period were considered to have had "washout" pregnancies, because the drug likely had washed out of their system prior to conception. Fetal exposure to Tysabri likely occurred in those who took the drug up to their last period.

For the study on risks to mothers, women taking Tysabri were compared against 350 pregnancies in MS patients who either took no medication or who were on another type of drug called interferon beta.

The risk of MS relapse during pregnancy was three times higher for women on Tysabri, the researchers found. About 37 percent of Tysabri users had at least one relapse, compared to 10 percent of the control group.

But further analysis found that only "washout" pregnancies were associated with having an MS relapse during pregnancy, said Dr. Ruth Ann Marrie, director of the Multiple Sclerosis Clinic at the University of Manitoba. She co-authored an editorial accompanying the studies, which were published Feb. 7 in the journal Neurology.

But while staying on Tysabri up to conception reduced a mom's risk of MS relapse, it also can exact a price when it comes to fetal health, the second study concluded.

Researchers found that for pregnancies exposed to Tysabri in the first trimester, the risk of miscarriage was four times higher than pregnancies where a mother took interferon beta or nothing for their MS.

The research team noted that the miscarriage rate of 17 percent for Tysabri-exposed pregnancies was close to the overall rate for the general population, which was 14 percent.

"To me, it's saying that for anybody who is exposed to Tysabri there was lower birth weight and lower length at birth, and higher chance of spontaneous abortion," said Dr. Asaff Harel, a neurologist with Lenox Hill Hospital in New York City. "That's even people who were just exposed at the very, very beginning of the pregnancy."

Because of this, "I'm apprehensive about the risk of continuing Tysabri past conception," Harel said. "On the flip side, I do agree with getting women back on the medication as soon as possible, especially if they've had a severe course of disease."

Marrie said the decision will likely come down to each woman's medical history.

"I don't think we can say every woman with MS should continue natalizumab up to pregnancy in an absolute way," Marrie said. "If you're thinking about pregnancy, let's plan it and talk about what the pros and cons are for you."

The list of medical terms above are retrieved automatically from the article.

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